In high-performance chain drive systems for mining machinery, it is known to use separate main and auxiliary drives usually at the end regions of a working face. Each drive may have an asynchronous electric motor and a gearing unit, usually spur gearing, which drives a sprocket wheel around which the chain is entrained. The drive or traction chain tends to be subject to uneven loads and the electrical power cannot be used efficiently under these circumstances without some compensation. The asynchronous motors are unregulated but the common drive chain causes the drives to be synchronised. Sometimes, the total length of the chain is sub-divided into sections with different elongations and because of the forced synchronisation of the drives there is imbalance in the loading of the drives. Asynchronous motors are sensitive to changes in loading and even small imbalance will caused one or both of the motors to run under partial loading. Under extreme conditions, one of the motors can even act as a generator. In order to utilise the electrical power efficiently and to distribute loading evenly between the drives it is known to employ an over-ride gearing or drive usually with planetary gearing which is driven in accordance with load or rotational speed see Gluckauf 1979 p. 456-460; 1981 p. 386-388; 1986 p. 875-880 and 849-859 and 1974 p. 239-243. A general object of the invention is to provide an improved drive system and gearing unit.